(1) Field of the Invention
The present invention relates to a system for exhaust gas purification, capable of effectively purifying the harmful substances present in the exhaust gas emitted from an internal combustion engine, particularly the hydrocarbons discharged from the engine in a large amount during the cold start.
(2) Description of Related Art
Active researches and developments have heretofore been made on systems for exhaust gas purification, used for purifying the harmful substances present in exhaust gas from automobile engine, such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and the like. In recent years, as the regulation on exhaust gas has become stricter, the purification of HC during engine start (cold start) has become an important technical task.
The reason is as follows. During the cold start of engine when the temperature of exhaust gas from engine is low, the catalyst disposed in exhaust pipe of engine does not reach its light-off temperature and has a low purification ability. Moreover, during this period, as compared with the period of continuous operation of engine, a large amount of HC is discharged from the engine. As a result, the HC discharged during the cold start occupies a large proportion of the total harmful substances discharged from the engine and needs to be removed.
As one means for achieving the above-mentioned technical task (the purification of HC during engine start), there was proposed a system for exhaust gas purification disposed in the exhaust pipe of an internal combustion engine, comprising (1) an adsorbent comprising a monolithic carrier and an adsorbent component (e.g. zeolite) having an adsorptivity for HC, loaded on the carrier and (2) a catalyst provided downstream of the adsorbent (1). This adsorbent is intended to adsorb the unburnt HC discharged from the engine in a large amount during the cold start, temporarily from the start of catalyst heating to the start of catalyst light-off.
As the adsorbent used in such a system for exhaust gas purification, an adsorbent having a circular sectional shape is known. Also, a system using an adsorbent having a elliptical sectional shape is disclosed in U.S. Pat. No. 5,619,853.
The HC adsorbed by an adsorbent begins to be desorbed as the temperature of the adsorbent increases owing to the heat of exhaust gas, etc. In an adsorbent having a circular sectional shape, such as mentioned above, since the distance from the sectional center to the sectional periphery is constant, the amount of the exhaust gas flowing through the peripheral area is about the same at any point of peripheral area and the temperature elevation rate is about the same as well at any point of peripheral area. As a result, in an adsorbent having a circular sectional shape, the whole peripheral area reaches the HC desorption start temperature simultaneously and the HC adsorbed by the adsorbent is desorbed in a large amount in a relatively short time.
Also in the system using an adsorbent having an elliptical sectional shape, described in U.S. Pat. No. 5,619,853, the amount of the exhaust gas flowing through the adsorbent is made the same at any sectional point, by the use of a diverter provided upstream of the adsorbent; therefore, the whole section shows about the same temperature elevation and reaches the HC desorption start temperature simultaneously; consequently, the HC adsorbed by the adsorbent is desorbed in a large amount in a relatively short time.
Such desorption of HC from adsorbent in large amount in short time has caused, in some cases, a problem that the HC amount in exhaust gas exceeded the HC amount which can be purified by a catalyst in the same short length of time. As a result, there has occurred, in some cases, a problem that the purification ratio for the HC generated during cold start is low.
The present invention has been completed in view of the above-mentioned problems of the prior art. The object of the present invention is to provide a system for exhaust gas purification, in which the adsorbent can desorb the adsorbed HC in a longer time and the HC generated during cold start can be removed at a higher purification ratio.
According to the present invention there is provided a system for exhaust gas purification disposed in the exhaust pipe of an internal combustion engine, which comprises:
an adsorbent comprising a monolithic carrier and an adsorbent component having a hydrocarbon adsorptivity, loaded on the carrier, and
a catalyst comprising a monolithic carrier and a catalyst component loaded on the carrier, having a purifiability for the harmful substances present in the exhaust gas emitted from the engine, the catalyst being provided downstream of the adsorbent in the flow direction of the exhaust gas,
in which system the adsorbent has a sectional shape satisfying the following relation:
1.2xe2x89xa6(major axis)/(minor axis)xe2x89xa66.5 